26 f, recent blood work showed low phosphorous level of 2.4 mg/dl (2.5-4.5 normal range) + leukocytes & microscopic blood in urine. Possible causes?

Low Phosphorous. You have 2 unrelated problems. You urine findings are compatible with a urinary tract infection. Sent off urine for culture to see if need antibiotics. Your low phosphorous (p) level, is not really that low. Repeat it and have the doctor order a calcium level as well. If you have a low p with a high ca, you may have primary hyperparathryoidism. A low p may also mean you eat a low protein diet.
Not worried. I'm going to assume you have no symptoms beyond what you described & these are your only abnormal labs. The just-below-reference-range phosphorus means nothing at all (ranges are set for 90% of asymptomatic couch potatoes). Get followup on the urine sediment if you're asymptomatic; it could just be contamination. If your dr. Isn't concerned, i'm not either.